It would be if such an event actually showed up in the event list for the radio button, which is appears not to.
Anyway, an answer was provided at another site shown below:
"RadioButton.TabStop property is infrastructure and is set dynamically at runtime to allow tabbing directly into checked radio and out of radio group again.
The best option to prevent tabstopping to radiobuttons is to select Enter event handler for all radios and call SelectNextControl method to auto select next control in tab order."
So I added this code in the ENTER event:
ctl = (Control)sender;
this.SelectNextControl(ctl, true, true, true, true);